Bodies of earth-boring tools, such as earth-boring rotary drill bits, may be formed from a particle-matrix composite material. Such particle-matrix composite materials include particles of hard material such as, for example, tungsten carbide dispersed throughout a metal matrix material (often referred to as a “binder” material). Particle-matrix composite materials exhibit relatively higher erosion and wear resistance relative to steel and other metal materials.
There are three primary types of tungsten carbide particles most often used in earth-boring tools, those being cast tungsten carbide particles, sintered tungsten carbide particles, and macrocrystalline tungsten carbide particles. The tungsten carbide system includes the two stoichiometric compounds of monotungsten carbide (WC) and ditungsten carbide (W2C), as well as a continuous range of mixtures there between of these two compounds. Cast tungsten carbide particles generally include a eutectic mixture of the monotungsten carbide and ditungsten carbide stoichiometric compounds. Sintered tungsten carbide particles generally include relatively smaller particles of monotungsten carbide (WC) bonded together by a matrix material. Cobalt and cobalt alloys are often used as matrix materials in sintered tungsten carbide particles. Sintered tungsten carbide particles may be formed by mixing together a first powder that includes the tungsten carbide particles and a second powder that includes the relatively smaller cobalt particles. The powder mixture is formed in a “green” state. The green powder mixture then is sintered at a temperature near the melting temperature of the cobalt particles to form a matrix of cobalt material surrounding the tungsten carbide particles to form particles of sintered tungsten carbide. Finally, macrocrystalline tungsten carbide particles generally comprise single crystals of monotungsten carbide (WC).
Typically, the body of an earth-boring drill bit is formed by providing particulate tungsten carbide material in a mold cavity having a shape corresponding to the body of the drill bit to be formed, melting a metal matrix material, such as a copper-based alloy, and infiltrating the particulate tungsten carbide material with the molten metal matrix material. After infiltration, the molten metal matrix material is allowed to cool and solidify. The resulting bit body may then be removed from the mold. Cast tungsten carbide particles are often used for at least a portion of the particulate tungsten carbide material in such infiltration processes.
During such infiltration processes, the cast tungsten carbide particles may interact chemically with the surrounding metal matrix material at the elevated temperatures at which infiltration is carried out. For example, atomic diffusion may occur between the cast tungsten carbide particles and the metal matrix material during infiltration. As a result, carbon and tungsten may diffuse out from the cast tungsten carbide particles and into the metal matrix material during infiltration, resulting in the formation of relatively small deposits or regions of unintended metal carbide satellite materials (such as, for example, so-called “eta-phase” carbides or carbides having a composition of the form M6C, where M is a metal) within the matrix material proximate the cast tungsten carbide particles. In these metal carbide satellite materials, the metal may be contributed by the matrix and the carbon may be contributed by the tungsten carbide particles. When a body of an earth-boring tool that includes such small metal carbide phases surrounding cast tungsten carbide particles cracks during use, the cracks may exhibit a tendency to propagate through the metal matrix material along a pathway that appears to follow the small metal carbide phases surrounding the cast tungsten carbide particles.